(1) Field of the Invention
The present invention is related to a periodical switching device which is optionally capable of periodically and continuously adjusting the operation time ranging from a switch closure of a short period of time through up to a continuous current-carrying state.
(2) Description of the Prior Art
Numerous kinds of electric devices have so far been developed, and many switching devices adapted to control such electric devices have been proposed depending upon the development of the electric devices. Among many electric devices, some are equipped with a switch which permits the operation time to be optionally varied at the will of a user. For example, an electronic range employing a magnetron, when used according to its purpose, features that the cooking can be carried out under quick heating at high temperatures. Depending upon the kinds of cooking, however, the quick heating at high temperatures may not be necessarily suitable. In such a case, the electronic range must have a function to reduce its output. An example which is not suitable for the quick heating at high temperatures may be the thawing cooking. A generally employed method of controlling the output of the electronic ranges consists of periodically interrupting the circuit for oscillating the magnetron to reduce the rate of oscillating time with respect to the power-producing state attained when the magnetron is continuously oscillated, within a predetermined period of cooking and heating time.
Conventional switching devices used for the aforementioned object will include, for example, a switching device shown in an expansion view of the diagram (a) of FIG. 1, in which a cam plate of which one side of a diagonal line of the rectangle is high and the other side is low, is rolled to form a cylindrical cam 2' as shown in the diagram (b), and a lever follower 5' is caused to slide in the axial directions of the cylinder 2' to utilize the change in height of the spiral cam surface; and a switching device shown in FIG. 2, in which a U-shaped yoke 12' and a lever 5' are rotatably mounted on a mounting member 11', the U-shaped yoke 12' and the lever 5' are urged toward one side by springs 18 and 19, an electric motor is mounted on the U-shaped yoke 12' so as to rotate the cam 2', and the U-shaped yoke 12' is moved together with the electric motor by means of a cam 2" disposed on the lower side of the mounting member 11', in order that the time for opening and closing a microswitch 7 is varied by means of the lever 5'. In the former example, however, the lever follower 5.dbd. receives the forces not only in the operating direction of the switch but also in the axial direction of the cylindrical cam, whereby the operation of the lever follower becomes unreliable giving a cause of trouble. In the latter example employing a normally-closed switch, on the other hand, the contacting position of the lever 5' is gradually varied as the cam 2' rotates counter-clockwise, whereby the switch is opened and closed with slow movement. When the switch is operated with such a setup, however, the contacts are brought into contact or separated with slow motion, inviting the development of arcing across the contacts and causing the contact surfaces to be deteriorated, which is a serious defect. Furthermore, the switches incorporated in the switching devices have dispersed dynamic points which, when opened and closed at low speeds, directly reflect their dispersed quality, making it difficult to uniformalize the accuracy in control time.